Clip-style medical sensor and technique for using the same

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to medical devices and, more particularly, to sensors used for sensing physiological parameters of a patient.

2. Description of the Related Art

This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.

In the field of medicine, doctors often desire to monitor certain physiological characteristics of their patients. Accordingly, a wide variety of devices have been developed for monitoring many such physiological characteristics. Such devices provide doctors and other healthcare personnel with the information they need to provide the best possible healthcare for their patients. As a result, such monitoring devices have become an indispensable part of modem medicine.

One technique for monitoring certain physiological characteristics of a patient is commonly referred to as pulse oximetry, and the devices built based upon pulse oximetry techniques are commonly referred to as pulse oximeters. Pulse oximetry may be used to measure various blood flow characteristics, such as the blood-oxygen saturation of hemoglobin in arterial blood, the volume of individual blood pulsations supplying the tissue, and/or the rate of blood pulsations corresponding to each heartbeat of a patient. In fact, the “pulse” in pulse oximetry refers to the time varying amount of arterial blood in the tissue during each cardiac cycle.

Pulse oximeters typically utilize a non-invasive sensor that transmits light through a patient's tissue and that photoelectrically detects the absorption and/or scattering of the transmitted light in such tissue. One or more of the above physiological characteristics may then be calculated based upon the amount of light absorbed or scattered. More specifically, the light passed through the tissue is typically selected to be of one or more wavelengths that may be absorbed or scattered by the blood in an amount correlative to the amount of the blood constituent present in the blood. The amount of light absorbed and/or scattered may then be used to estimate the amount of blood constituent in the tissue using various algorithms.

Conventional pulse oximetry sensors are either disposable or reusable. In many instances, it may be desirable to employ, for cost and/or convenience, a reusable pulse oximeter sensor. Reusable sensors are typically semi-rigid or rigid devices that may be clipped to a patient. Unfortunately, reusable sensors may be uncomfortable for the patient for various reasons. For example, sensors may have angled or protruding surfaces that, over time, may cause discomfort. In addition, reusable pulse oximeter sensors may pose other problems during use. For example, lack of a secure fit may allow light from the environment to reach the photodetecting elements of the sensor, thus causing inaccuracies in the resulting measurement.

Because pulse oximetry readings depend on pulsation of blood through the tissue, any event that interferes with the ability of the sensor to detect that pulsation can cause variability in these measurements. A reusable sensor should fit snugly enough that incidental patient motion will not dislodge or move the sensor, yet not so tight that normal blood flow to the tissue is disrupted. As sensors are worn for several hours at a time, an overly tight fit may cause local exsanguination of the tissue around the sensor. Exsanguinated tissue, which is devoid of blood, shunts the sensor light through the tissue, resulting in increased measurement errors.

SUMMARY

Certain aspects commensurate in scope with the originally claimed invention are set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of certain forms that the invention might take and that these aspects are not intended to limit the scope of the invention. Indeed, the invention may encompass a variety of aspects that may not be set forth below.

There is provided a sensor that includes: a sensor body having a first portion and a second portion; a spring adapted to bias the first portion towards the second portion; a stopping element adapted to establish a minimum distance between the first portion and the second portion; and at least one sensing element disposed on the sensor body.

There is provided a sensor that includes: a sensor body having a first portion, a second portion; a spring adapted to bias the first portion towards the second; a substrate disposed on at least one of the first portion or the second portion, wherein the substrate is adapted to move with at least one degree of freedom relative to the sensor body; and at least one sensing element disposed on the substrate.

There is also provided a pulse oximetry system that includes: a pulse oximetry monitor and a pulse oximetry sensor adapted to be operatively coupled to the monitor, the sensor comprising: a sensor body having a first portion and a second portion; a spring adapted to bias the first portion towards the second portion; a stopping element adapted to establish a minimum distance between the first portion and the second portion; and at least one sensing element disposed on the sensor body.

There is also provided a method of operating a sensor that includes: biasing a first portion and a second portion of a sensor body towards one another with a spring; and establishing a minimum distance between the first portion and the second portion with a stopper disposed on the sensor body.

There is also provided a method of manufacturing a sensor that includes: providing a sensor body having a first portion and a second portion; providing a spring adapted to bias the first portion towards the second portion; providing a stopping element adapted to establish a minimum distance between the first portion and the second portion; and providing at least one sensing element disposed on the sensor body.

BRIEF DESCRIPTION OF THE DRAWINGS

Advantages of the invention may become apparent upon reading the following detailed description and upon reference to the drawings in which:

<figref idrefs="DRAWINGS">FIG. 1A</figref> illustrates a perspective view of an exemplary sensor with a stopper and a flat spring according to the present invention;

<figref idrefs="DRAWINGS">FIG. 1B</figref> illustrates the sensor of <figref idrefs="DRAWINGS">FIG. 1A</figref> applied to a patient earlobe according to the present invention;

<figref idrefs="DRAWINGS">FIG. 2A</figref> illustrates a perspective view of an exemplary sensor with a rigid bar according to the present invention;

<figref idrefs="DRAWINGS">FIG. 2B</figref> illustrates a cross-sectional view of the open sensor of <figref idrefs="DRAWINGS">FIG. 2A</figref>;

<figref idrefs="DRAWINGS">FIG. 2C</figref> illustrates a cross-sectional view of the sensor of <figref idrefs="DRAWINGS">FIG. 2A</figref> applied to a patient's earlobe;

<figref idrefs="DRAWINGS">FIG. 2D</figref> illustrates a cross-sectional view of a sensor including an adjustable bar;

<figref idrefs="DRAWINGS">FIG. 3A</figref> illustrates a cross-sectional view of an open exemplary sensor with a stopper within a hinge according to the present invention;

<figref idrefs="DRAWINGS">FIG. 3B</figref> illustrates a cross-sectional view of the sensor of <figref idrefs="DRAWINGS">FIG. 3A</figref> applied to a patient's earlobe;

<figref idrefs="DRAWINGS">FIG. 4A</figref> illustrates a cross sectional view of an exemplary sensor with a strap according to the present invention;

<figref idrefs="DRAWINGS">FIG. 4B</figref> illustrates a cross-sectional view of the sensor of <figref idrefs="DRAWINGS">FIG. 4A</figref> applied to a patient's earlobe;

<figref idrefs="DRAWINGS">FIG. 4C</figref> illustrates a cross-sectional view of a sensor with an adjustable strap;

<figref idrefs="DRAWINGS">FIG. 4D</figref> illustrates a cross sectional view of an alternative embodiment of the sensor of <figref idrefs="DRAWINGS">FIG. 4A</figref> with an offset emitter and detector;

<figref idrefs="DRAWINGS">FIG. 5A</figref> illustrates a cross sectional view of an exemplary sensor with pivoting heads according to the present invention.

<figref idrefs="DRAWINGS">FIG. 5B</figref> illustrates a cross-sectional view of the sensor of <figref idrefs="DRAWINGS">FIG. 5A</figref> applied to a patient's earlobe; and

<figref idrefs="DRAWINGS">FIG. 6</figref> illustrates a pulse oximetry system coupled to a multi-parameter patient monitor and a sensor according to embodiments of the present invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

One or more specific embodiments of the present invention will be described below. In an effort to provide a concise description of these embodiments, not all features of an actual implementation are described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure.

In accordance with the present technique, motion-resistant pulse oximetry sensors are provided that reduce measurement error by applying limited and uniform pressure to the optically probed tissue. A clip-style sensor for pulse oximetry or other spectrophotometric uses is provided that has a compliant material disposed on the sensor to distribute the spring force of the clip to the tissue evenly when the sensor is applied to a patient. The clip-style sensor may also have a stopper that prevents the two portions of the clip from applying an excess of pressure to the patient's tissue. Alternatively, the clip-style sensor may allow the light emitting and detecting components of the sensor to tilt or otherwise move to accommodate the patient's tissue and to prevent overly tight gripping at the sensor placement site.

Pulse oximetry sensors are typically placed on a patient in a location that is normally perfused with arterial blood to facilitate measurement of the desired blood characteristics, such as arterial oxygen saturation measurement (SpO₂). The most common sensor sites include a patient's fingertips, toes, earlobes, or forehead, and clip-style sensors are most commonly used on patient digits, earlobes, or nose bridges. Regardless of the placement of the sensor <b>10</b>, the reliability of the pulse oximetry measurement is related to the accurate detection of transmitted light that has passed through the perfused tissue. Hence, a sensor <b>10</b> that fits a patient securely may reduce movement of the sensor and/or infiltration of light from outside sources into the sensor, which may lead to more accurate pulse oximetry measurements.

There are several factors that may influence the tightness with which a sensor may grip a patient's tissue. It is desirable to affix the sensor <b>10</b> to the patient in a manner that does not exsanguinate the tissue, but that provides sufficient pressure to squeeze out excess venous blood. Excess venous blood congestion in the optically probed tissue may influence the relationship between the modulation ratio of the time-varying light transmission signals of the wavelengths transmitted and SpO₂. As venous blood has an increased concentration of deoxyhemoglobin as compared to arterial blood, its contribution to the pulse oximetry measurement may shift the wavelength of the detected light. Thus, the pulse oximetry sensor may measure a mixed arterial-venous oxygen saturation and detect differences in signal modulations unrelated to the underlying SpO₂level. It is therefore desirable to reduce the contribution of excess venous blood to the pulse oximetry measurement by clipping a sensor to a patient's tissue with enough spring force to squeeze out excess venous blood.

On the other hand, a patient's tissue may suffer if clipped too tightly by a pulse oximetry sensor. In addition to causing patient discomfort, a sensor with excess gripping force in a hinge spring or other closing mechanism may squeeze both arterial and venous blood from a patient's tissue, causing the tissue to become exsanguinated. Light from a sensor's emitter that passes through such exsanguinated tissue may not be modulated by arterial blood, which may cause the resulting SpO₂measurements to be artificially low. Thus, it is desirable to clip a sensor <b>10</b> to a patient's tissue tightly enough to reduce the amount of venous blood congestion, but not so tightly as to interfere with arterial blood perfusion.

In accordance with the present techniques, examples of clip-style sensors that apply limited, uniform pressure to a patient's tissue are disclosed. An exemplary sensor <b>10</b>A adapted for use on a patient's earlobe is illustrated in <figref idrefs="DRAWINGS">FIG. 1A</figref>. The sensor has a first portion <b>12</b> and a second portion <b>14</b> that are applied to opposite sides of an earlobe. The sensor body <b>16</b> includes a flat spring <b>18</b> that may be used to connect the first portion <b>12</b> and the second portion <b>14</b>. The first portion <b>12</b> and the second portion <b>14</b> may have a rigid outer layer <b>20</b>.

The sensor <b>10</b>A may also include a stopper <b>22</b> that limits the distance that the first portion <b>12</b> and the second portion <b>14</b> may move towards one another. Generally, it is envisioned that the stopper <b>22</b> be configured to allow the first portion <b>12</b> to move towards the second portion <b>14</b> such that they are not able to move past a minimum distance from one another that permits the sensor <b>10</b>A to securely grip a patient's tissue. Such a minimum distance may generally be determined by the desired sensor placement site (e.g. nose, earlobe, or digit) and the size of the patient (e.g. child or adult). As the sensor <b>10</b>A is applied to the patient's earlobe <b>24</b>, the stopper <b>22</b> absorbs part of the spring force of the flat spring <b>18</b> to prevent the sensor <b>10</b>A from gripping the tissue so tightly as to cause exsanguinations or discomfort. The stopper <b>22</b> may be permanently attached to the sensor body <b>16</b>, or may be removable. In certain embodiments, the stopper may be a plug.

In an alternate embodiment, <figref idrefs="DRAWINGS">FIG. 2A</figref> depicts a perspective side view of a sensor <b>10</b>B with a permanently attached rigid bar <b>30</b> acting as a stopper between a first portion <b>32</b> and a second portion <b>34</b> of a sensor body <b>36</b>. An emitter <b>26</b> is disposed on the first portion <b>32</b> and a detector <b>26</b> is disposed on the second portion <b>34</b>. The rigid bar <b>30</b> is permanently attached to the first portion <b>32</b> and moves away from the second portion <b>34</b> during the opening of the sensor <b>10</b>B, as shown in the cross-sectional view of the open sensor <b>10</b>B in <figref idrefs="DRAWINGS">FIG. 2B</figref>. However, it should be understood that the rigid bar <b>30</b> may alternatively be disposed on the second portion <b>34</b> in other embodiments. The rigid bar <b>30</b> as depicted is disposed on the first portion <b>32</b> of the sensor <b>10</b>B in a region of the sensor body <b>36</b> that is free of intervening tissue when the sensor <b>10</b>B is applied an earlobe <b>38</b>, as shown in <figref idrefs="DRAWINGS">FIG. 2C</figref>. As the sensor <b>10</b>B is closed, the rigid bar <b>30</b> contacts the second portion <b>34</b> and prevents further biasing of the first portion <b>32</b> towards the second portion <b>34</b>. The first portion <b>32</b> and the second portion <b>34</b> may be connected by a hinge <b>40</b> with a spring <b>42</b>. Thus, the rigid bar <b>30</b> restricts the range of motion of the hinge <b>40</b>, such that the hinge <b>40</b> may only move the first portion <b>32</b> and the second portion <b>34</b> toward one another to a certain degree. Thus, the maximum spring force applied to the tissue is limited because the rigid bar <b>30</b> limits the force that the first portion <b>32</b> and the second portion <b>34</b> may exert against the earlobe <b>38</b>.

When the sensor <b>10</b>B is applied to the patient's earlobe <b>38</b>, as shown in <figref idrefs="DRAWINGS">FIG. 2C</figref>, a resilient pad <b>44</b> absorbs part of the force of the spring <b>42</b> and distributes the remaining spring force to the earlobe <b>38</b> along the tissue-contacting surface of the sensor <b>10</b>B. Thus, the total compression resistance of the resilient material is generally less than the force of the spring <b>42</b>. The resilient pad may be any shock-absorbing material, including foam, silicone, or rubber. The sensor <b>10</b>B thereby evenly distributes a limited force to the patient's tissue through use of a resilient pad <b>44</b>, which spreads the force along the tissue-contacting surface.

In an alternate embodiment, depicted in <figref idrefs="DRAWINGS">FIG. 2D</figref>, the sensor <b>10</b>B may include an adjustable bar <b>31</b> that may be threaded through an opening (not shown) in the sensor body <b>36</b>. Thus, the length of the adjustable bar <b>31</b> may be increased by threading more length of the adjustable bar <b>31</b> through the sensor body <b>36</b>. In such an embodiment, the minimum distance between the first portion <b>32</b> and the second portion <b>34</b> may be increased to accommodate the tissue of larger patients. Alternatively, smaller patients may require adjustment of the adjustable bar <b>31</b> such that more of the adjustable bar is threaded outside the sensor body <b>36</b> (i.e. not in the region between the first portion <b>32</b> and the second portion <b>34</b>). Additionally, the sensor <b>10</b>B may be applied to the patient, and a healthcare worker may adjust the length of the adjustable bar <b>31</b> until a desired amount of pressure on the tissue is achieved. In certain embodiments, the adjustable bar may be further secured by a nut <b>33</b> or other holding mechanism.

It is also envisioned that spring force of a hinge may be restricted by other mechanical structures. For example, in an alternative embodiment shown in <figref idrefs="DRAWINGS">FIG. 3A</figref> and <figref idrefs="DRAWINGS">FIG. 3B</figref>, a sensor <b>10</b>C has a stopper <b>46</b> that is disposed within the mechanism of a hinge <b>48</b> to restrict rotational motion, thus preventing the hinge <b>48</b> from exerting maximum pressure to the tissue when sensor <b>10</b>C is applied to a patient's earlobe <b>58</b>. The stopper <b>46</b> may be a rigid material that is designed to mechanically block the motion of the hinge <b>48</b>.

As depicted, the emitter <b>50</b> and the detector <b>52</b> are disposed on a thin substrate <b>54</b>. The substrate <b>54</b> may be any suitable material, including plastic or woven cloth, and may be rigid or flexible. The substrate <b>54</b> may be disposed on the tissue-contacting side of a resilient pad <b>56</b>. In certain embodiments, it may be advantageous to employ a flexible substrate <b>54</b>, which may conform more closely to a patient's tissue when the sensor <b>10</b>C is applied. In other embodiments, a more rigid substrate <b>54</b> may absorb more of the spring force of the hinge <b>48</b>, and thus may prevent the sensor <b>10</b>K from exerting excess pressure on the tissue.

Alternatively, as shown by the embodiment illustrated in <figref idrefs="DRAWINGS">FIGS. 4A-D</figref>, a sensor <b>10</b>D may have a flexible but inelastic strap <b>60</b>, such as a plastic or metal strap, disposed on the handle end <b>62</b> of the sensor body, connecting the first portion <b>64</b> and the second portion <b>66</b>. When the sensor <b>10</b>D is open, the strap <b>60</b> is slack. When the sensor <b>10</b>D is closed, such as when the sensor <b>10</b>D is applied to a patient, as shown in <figref idrefs="DRAWINGS">FIG. 4B</figref>, the strap <b>60</b> is drawn taut, thus preventing the hinge <b>68</b> from moving the first portion <b>64</b> and the second portion <b>66</b> closer than a distance dictated by the length of the strap <b>60</b>.

As depicted, the sensor <b>10</b>D has resilient pads <b>70</b> disposed on the tissue-contacting sides of the first portion <b>64</b> and the second portion <b>66</b> of a sensor. The use of a resilient pad <b>70</b> on both the first portion <b>64</b> and the second portion <b>66</b> provides greater compression resistance against the spring force of the hinge <b>68</b> than only a single resilient pad. Additionally, the spring force is evenly distributed along the tissue-contacting surface that holds both the emitter <b>72</b> and the detector <b>74</b> against the tissue. Thus, a sensor <b>10</b>D may be used in conjunction with a relatively strong spring. This may be advantageous in situations in which an ambulatory patient may require the sensor <b>10</b>D to fit securely enough to withstand dislodgement in response to everyday activity.

In an alternate embodiment, <figref idrefs="DRAWINGS">FIG. 4C</figref> illustrates a sensor <b>10</b>D with an adjustable strap <b>61</b>. The adjustable strap <b>61</b> may be threaded through an opening (not shown) in the sensor body. When an appropriate length of the adjustable strap is disposed between the first portion <b>64</b> and the second portion <b>66</b> to provide the desired pressure on a patient's tissue, the adjustable strap <b>61</b> may be held in place by a clamp <b>63</b>. As more length of the adjustable strap <b>61</b> is released into the region between the first portion <b>64</b> and the second portion <b>66</b>, the sensor <b>10</b>D is able to close more tightly over the tissue. Alternatively, a healthcare worker may pull the adjustable strap <b>61</b> through the sensor body such that the length of adjustable strap <b>61</b> between the first portion <b>64</b> and the second portion <b>66</b> is decreased, and as a result the sensor <b>10</b>D would exert less pressure on the tissue.

Clip-style sensors as provided herein are often used on a patient's earlobes, which may have fewer vascular structures as compared to a digit. To maximize the transmission of light through well-perfused capillary structures, an alternative embodiment of the sensor <b>10</b>D is depicted in which the emitter <b>72</b> and detector <b>74</b> are offset from each other, so that they are not directly opposite. This causes the light emitted by the emitter <b>72</b> to pass through more blood-perfused tissue to reach the detector <b>74</b>. As such, the light has a greater opportunity to be modulated by arterial blood in a manner which relates to a blood constituent. <figref idrefs="DRAWINGS">FIG. 4D</figref> illustrates that the configuration of the sensor <b>10</b>D provides a longer light transmission path through the tissue, as indicated by arrow <b>75</b>.

<figref idrefs="DRAWINGS">FIG. 5A</figref> and <figref idrefs="DRAWINGS">FIG. 5B</figref> depict an embodiment of a sensor <b>10</b>E in which part of the spring force of a hinge <b>76</b> is absorbed by pivoting heads <b>78</b>, upon which an emitter <b>80</b> and a detector <b>82</b> are disposed. The pivoting heads <b>78</b> are disposed on a first portion <b>84</b> and a second portion <b>86</b> of the sensor <b>10</b>E. The first portion <b>84</b> and the second portion <b>86</b> are connected by the hinge <b>76</b>. Pivoting heads are disposed on the tissue-contacting side of the first portion <b>84</b> and the second portion <b>86</b>. As <figref idrefs="DRAWINGS">FIG. 5B</figref> illustrates, the pivoting heads <b>78</b> may tilt relative to the sensor body <b>88</b> in order to accommodate the contours of the patient's tissue. In certain embodiments, the pivoting heads <b>78</b> may also include resilient pads (not shown) that distribute the spring force of the hinge <b>76</b> along the tissue-contacting surface of the sensor <b>10</b>E. In other embodiments, the sensor <b>10</b>E may also include a stopper or stopping mechanism as described herein.

In an alternate embodiment (not shown), an adhesive material is applied to the tissue-contacting surface of the sensor <b>10</b> to enhance the securing of the sensor <b>10</b> to the tissue. The use of an adhesive material may improve the contact of the sensor to the appendage, and limit the susceptibility to motion artifacts. In addition, the likelihood of a gap between the sensor body and the skin is avoided.

In certain embodiments, it is contemplated that the spring force of the hinge (e.g. <b>40</b>, <b>48</b>, <b>68</b>, or <b>78</b>) or other closing mechanism, such as a flat spring (e.g. flat spring <b>18</b>), has sufficient pressure so that it exceeds the typical venous pressure of a patient, but does not exceed the diastolic arterial pressure. A sensor <b>10</b> that applies a pressure greater than the venous pressure will squeeze excess venous blood from the optically probed tissue, thus enhancing the sensitivity of the sensor to variations in the arterial blood signal. Since the pressure applied by the sensor is designed to be less than the arterial pressure, the application of pressure to the tissue does not interfere with the arterial pulse signal. Typical venous pressure, diastolic arterial pressure and systolic arterial pressure are less than 10-35 mmHg, 80 mmHg, and 120 mmHg, respectively. These pressures may vary because of the location of the vascular bed and the patient's condition. In certain embodiments, the sensor may be adjusted to overcome an average pressure of 15-30 mmHg. In other embodiments, low arterial diastolic blood pressure (about 30 mmHg) may occur in sick patients. In such embodiments, the sensor <b>10</b> may remove most of the venous pooling with light to moderate pressure (to overcome about 15 mmHg). It is contemplated that removing venous blood contribution without arterial blood exsanguination may improve the arterial pulse signal.

The exemplary sensors described above, illustrated generically as a sensor <b>10</b>, may be used in conjunction with a pulse oximetry monitor <b>90</b>, as illustrated in <figref idrefs="DRAWINGS">FIG. 6</figref>. It should be appreciated that the cable <b>92</b> of the sensor <b>10</b> may be coupled to the monitor <b>90</b> or it may be coupled to a transmission device (not shown) to facilitate wireless transmission between the sensor <b>10</b> and the monitor <b>90</b>. The monitor <b>90</b> may be any suitable pulse oximeter, such as those available from Nellcor Puritan Bennett Inc. Furthermore, to upgrade conventional pulse oximetry provided by the monitor <b>90</b> to provide additional functions, the monitor <b>90</b> may be coupled to a multi-parameter patient monitor <b>94</b> via a cable <b>96</b> connected to a sensor input port or via a cable <b>98</b> connected to a digital communication port.

The sensor <b>10</b> includes an emitter <b>100</b> and a detector <b>102</b> that may be of any suitable type. For example, the emitter <b>100</b> may be one or more light emitting diodes adapted to transmit one or more wavelengths of light in the red to infrared range, and the detector <b>102</b> may be a photodetector selected to receive light in the range or ranges emitted from the emitter <b>100</b>. For pulse oximetry applications using either transmission or reflectance type sensors, the oxygen saturation of the patient's arterial blood may be determined using two or more wavelengths of light, most commonly red and near infrared wavelengths. Similarly, in other applications, a tissue water fraction (or other body fluid related metric) or a concentration of one or more biochemical components in an aqueous environment may be measured using two or more wavelengths of light, most commonly near infrared wavelengths between about 1,000 nm to about 2,500 nm. It should be understood that, as used herein, the term “light” may refer to one or more of infrared, visible, ultraviolet, or even X-ray electromagnetic radiation, and may also include any wavelength within the infrared, visible, ultraviolet, or X-ray spectra.

The emitter <b>100</b> and the detector <b>102</b> may be disposed on a sensor body <b>104</b>, which may be made of any suitable material, such as plastic, foam, woven material, or paper. Alternatively, the emitter <b>100</b> and the detector <b>102</b> may be remotely located and optically coupled to the sensor <b>10</b> using optical fibers. In the depicted embodiments, the sensor <b>10</b> is coupled to a cable <b>92</b> that is responsible for transmitting electrical and/or optical signals to and from the emitter <b>100</b> and detector <b>102</b> of the sensor <b>10</b>. The cable <b>92</b> may be permanently coupled to the sensor <b>10</b>, or it may be removably coupled to the sensor <b>10</b>—the latter alternative being more useful and cost efficient in situations where the sensor <b>10</b> is disposable.

The sensor <b>10</b> may be a “transmission type” sensor. Transmission type sensors include an emitter <b>100</b> and detector <b>102</b> that are typically placed on opposing sides of the sensor site. If the sensor site is a fingertip, for example, the sensor <b>10</b> is positioned over the patient's fingertip such that the emitter <b>100</b> and detector <b>102</b> lie on either side of the patient's nail bed. In other words, the sensor <b>10</b> is positioned so that the emitter <b>100</b> is located on the patient's fingernail and the detector <b>102</b> is located 180° opposite the emitter <b>100</b> on the patient's finger pad. During operation, the emitter <b>100</b> shines one or more wavelengths of light through the patient's fingertip and the light received by the detector <b>102</b> is processed to determine various physiological characteristics of the patient. In each of the embodiments discussed herein, it should be understood that the locations of the emitter <b>100</b> and the detector <b>102</b> may be exchanged. For example, the detector <b>102</b> may be located at the top of the finger and the emitter <b>100</b> may be located underneath the finger. In either arrangement, the sensor <b>10</b> will perform in substantially the same manner.

Reflectance type sensors generally operate under the same general principles as transmittance type sensors. However, reflectance type sensors include an emitter <b>100</b> and detector <b>102</b> that are typically placed on the same side of the sensor site. For example, a reflectance type sensor may be placed on a patient's fingertip or forehead such that the emitter <b>100</b> and detector <b>102</b> lie side-by-side. Reflectance type sensors detect light photons that are scattered back to the detector <b>102</b>.

While the invention may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Indeed, the present techniques may not only be applied to measurements of blood oxygen saturation, but these techniques may also be utilized for the measurement and/or analysis of other blood constituents using principles of pulse oximetry. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the following appended claims.